Acousto-optic tuning of lasers

ABSTRACT

A semiconductor laser tuned with an acousto-optic modulator. The acousto-optic modulator may generate standing waves or traveling waves. When traveling waves are used, a second acousto-optic modulator may be used in a reverse orientation to cancel out a chirp created in the first acousto-optic modulator. The acousto-optic modulator may be used with standing-wave laser resonators or ring lasers.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation of U.S. patent application Ser. No. 16/298,873, filed Mar. 11, 2019, titled “Acousto-Optic Tuning of Lasers” (now U.S. Pat. No. 10,615,562), which is a continuation-in-part of U.S. patent application Ser. No. 14/636,058, filed Mar. 2, 2015, titled “Acousto-Optic Tuning of Lasers” (now U.S. Pat. No. 10,230,210), which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/947,067, entitled “Acousto-Optic Tuning of QCLs and ICLs,” filed Mar. 3, 2014, which applications are incorporated in their entirety here by this reference.

TECHNICAL FIELD

This invention relates to tuning quantum cascade lasers and inter-band cascade lasers.

BACKGROUND

Quantum cascade lasers (QCL) are lasers in which the gain spectrum is typically broader than approximately 5% of the central wavelength of the laser. In typical configurations (such as Fabry-Perot configurations, exemplified by FIG. 2 without the grating 14), i.e., with one facet of the QCL 10 with high reflectivity coating and the exit facet with controlled reflectivity anti-reflection coating 12, the QCLs 10 will produce very high power output, approaching power greater than 4 W at a wavelength of approximately 4.6 micrometer. However, as shown in FIG. 1, this output occurs in a bandwidth of approximately 250 nm around the central wavelength.

Such broadband operation is acceptable when the precise wavelength or the bandwidth of the output is not critical, for example, for directional infrared countermeasures (DIRCM) targeting, beacon, and illumination applications. On the other hand, there are a significant number of very important applications, where the laser output must be narrow band, for example, less than 1 nm wide, and tunable over some wavelength region. These applications include spectroscopy and sensors for detection of pollutants, chemical warfare agents, toxic gases and explosives. To obtain a “single frequency” output from a broadband gain spectrum laser such as that from a QCL, a wavelength dispersive element needs to be introduced within the laser cavity so that only one selected wavelength can resonate. Such dispersive elements include diffraction gratings 14 (FIGS. 2 and 3), prisms 18 (FIG. 4) and tunable or otherwise narrow band filters 20 (FIG. 5).

A key feature of all of these schemes is that mechanical motion is required to tune, i.e., change, the wavelength of the laser since the wavelength selection is dependent on the angle as shown in the FIGS. 2-5. All the techniques shown in these figures permit tuning of the laser wavelength over the entire gain spectrum (as long as the round trip optical gain exceeds total cavity losses). But, the tuning is slow because of the mechanical motion of a discrete, dispersive element (grating, prism or filter) and not appropriate for applications calling for ruggedness, such as for sensors that would be deployed in the field, carried by personnel or mounted on vehicles. There are many applications that require very rapid tuning because there is a need to obtain a complete spectrum of the object under examination in a very short time. Such applications include studies of time dependent combustion dynamics and explosion dynamics, time dependent spectral changes during chemical and biological reactions, rapid examination of an improvised explosive device (IED) in standoff detection mode and tracking the release of toxic gases.

There is yet another way of obtaining narrow linewidth output from an otherwise broadband QCL. This is the use of distributed feedback grating, which is embedded within the gain structure of the laser. Such lasers are useful because they are simple to fabricate and are rugged. However, tunability is quite limited around the design wavelength of the distributed Bragg grating. Typical tuning range for distributed feedback lasers (DFB) is limited to approximately 5 cm⁻¹ around the design wavelength of the grating. This is but a small fraction of the gain spectrum width of the QCL. The tuning can be carried out either by varying the QCL drive current or by changing the temperature of the QCL. In either case, no mechanical motion is required. The thermal tuning is slow while the electrical current driven tuning can be relatively fast. However, for obtaining broadband tuning, DFB lasers are inappropriate.

For the foregoing reasons there is a need for rugged, rapid broadband tuning of quantum cascade lasers.

SUMMARY

The present invention permits rapid broadband tuning of semiconductor lasers, such as quantum cascade lasers and interband cascade lasers, electronically without the use of any mechanical lotion for the wavelength selection, by utilizing an acousto-optic modulator (also called acousto-optic filter), thereby improving the ruggedness of the laser. The acousto-optic modulator ay generate traveling waves or standing waves. When using traveling waves, a second acousto-optic modulator may be used in the reverse orientation compared to the first acousto-optic modulator to cancel out any Doppler frequency shifts from the first acousto-optic modulator. The acousto-optic modulator can be used with standing-wave laser resonators or ring lasers.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph of a spectral output from a MWIR high power quantum cascade laser in Fabry-Perot configuration (no internal wavelength dispersion component).

FIG. 2 shows a schematic of a diffraction grating tuned broadband laser (Littrow configuration).

FIG. 3 shows a schematic of another diffraction grating tuned broadband laser (Littman-Metcalf configuration).

FIG. 4 shows a schematic of a prism tuned broadband laser.

FIG. 5 shows a schematic of a narrow bandwidth filter tuned broadband laser.

FIG. 6 shows a schematic of an embodiment of an acousto-optic modulator (filter) tuned quantum cascade laser.

FIG. 7 shows a schematic of another embodiment of an acousto-optic modulator (filter) tuned quantum cascade laser.

FIG. 8 shows a schematic of an embodiment of a standing wave acousto-optic modulator tuned quantum cascade laser.

FIG. 9 shows a schematic of another embodiment of a standing wave acousto-optic modulator tuned quantum cascade laser with optical output through zero order diffraction.

FIG. 10 is a graph showing discreet laser wavelength tuning as a function of acoustic driving frequency for a 1 cm long acousto-optic modulator.

FIG. 11 is a graph showing discreet laser wavelength tuning as a function of acoustic driving frequency for a 2 cm long acousto-optic modulator.

FIG. 12 shows a schematic of an embodiment of a traveling wave acousto-optic modulator tuned quantum cascade laser (Littrow configuration).

FIG. 13 shows a graph of a tuning curve for the acousto-optic modulator-controlled external cavity quantum cascade laser (EC QCL) setup.

FIGS. 14A-C show measured output spectra for the same QCL output wavelength at three different acousto-optic modulator frequencies.

FIG. 14D shows measured output spectra from FIGS. 14A-C overlapped onto a single graph for comparison.

FIG. 15 shows a graph of emission spectrum linewidth dependence on acousto-optic modulator central frequency.

FIGS. 16A-B show switching time data for the acousto-optic modulator-controlled EC QCL setup.

FIG. 17 shows a schematic defining the beam size D_(B) and sound wave propagation length to the beam L.

FIG. 18 shows a graph of simulated vs. experimental data for IPA (isopropyl alcohol) absorption.

FIG. 19 shows a schematic of an embodiment of a ring laser utilizing an acousto-optic modulator.

FIG. 20 shows a schematic of another embodiment of a ring laser utilizing an acousto-optic modulator.

FIG. 21 shows a schematic of another embodiment of a ring laser utilizing an acousto-optic modulator.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below in connection with the appended drawings is intended as a description of presently-preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention.

Emission wavelength of a single Fabry-Perot (FP) QCL chip with an anti-reflection-coated back facet is controlled with a dispersive element located outside the laser medium 102, i.e. using the external cavity (EC) approach. However, in contrast to traditional external cavity QCLs with a moving grating for example, the rapid wavelength tuning in the present invention is achieved using an electrically controlled acousto-optic modulator (AOM) 106. Compared with the DFB configuration, the present invention provides continuous tunability of the lasing wavelength over the entire gain bandwidth of the QCL; and compared with EC QCLs with a grating, which do provide continuous and broad tunability, the present invention has no moving parts.

An AOM 106 comprises a transparent material 107 having a piezoelectric transducer 112 attached at one end, and an acoustic absorber 114 attached at the opposite end. The piezoelectric transducer 112 creates a sound wave that is propagated through the transparent material 107 towards the acoustic absorber 114. In particular, high-frequency acoustic wave in AOMs 106 may be generated in a transparent material 107 (germanium in case of long wavelength infrared (LWIR) region) and this acoustic wave forms an index grating. The AOM 106 also has two opposing facets adjacent to each end of the opposing ends of the AOM 106. The two facets may have anti-reflection coatings 117 at the optical wavelength of interest on the two facets through which the laser radiation (i.e. beam) passes.

All commercially available AOMs, have anti-reflection coatings 117 at the optical wavelengths of interest on the two facets through which the laser radiation passes.

In some embodiments, two AOMs 106, 108, with opposed travelling acoustic waves 115 a, 115 b, respectively, may be used so that the Doppler frequency shift in the optical wave introduced by the first AOM 106 is cancelled by the complementary Doppler shift introduced by the second AOM 108. In other words, the second AOM 108 comprises a transparent material 109 having a first end and a second end, a second piezoelectric transducer 116 at the first end and a second acoustic absorber 118 at the second end, wherein the orientation of the second AOM 108 is reversed compared to the first AOM 106 as shown in FIGS. 6 and 7. In any embodiment in which a Doppler shift in the optical wave is anticipated, for example, when using travelling waves in the AOM, a second AOM 108 may be used in the opposite orientation as the first AOM 106 to counter the Doppler shift.

The geometry using two AOMs 106, 108 permits continuous tuning of the QCL (as opposed to discrete tuning in steps). The incident light waves emitted by the laser are deflected by the travelling acoustic gratings created in the AOMs. The angle of the deflection is controlled by the choices of optical and acoustic wavelengths according to equation 1 below:

$\begin{matrix} {{\sin\theta_{B}} = \frac{\lambda_{i} \cdot v_{a}}{2nV_{a}}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

where θ_(B) is the Bragg diffraction angle, λ₀ is the free space optical wavelength, n and V_(a) are the refractive index of and the acoustic velocity in the AO material, respectively, and v_(a) is acoustic frequency. The direction of the lasing is determined by the laser resonator cavity so the output laser wavelength can be electronically selected by changing the acoustic frequency of the AOM 106, thus changing the Bragg angle condition for the selected wavelength. Deflection efficiency of approximately 90 percent, comparable to that of traditional diffraction gratings, has been demonstrated for LWIR AOMs. The response time of the modulators is determined by the transit time of the acoustic wave across the material 107 and is consistent with high speed (rapid) measurements. Notice that once the system is aligned, there is no mechanical motion involved for tuning the wavelength of the output. The AOMs 106, 108 are aligned within the cavity and tuning of the output wavelengths is achieved by changing the driving acoustic frequency of the modulators according to Equation 1.

The deflection efficiency depends on modulator radiofrequency (RF) power generated by the radiofrequency generators 125, 126. The higher the RF power, the higher the efficiency, leading to higher feedback strength.

Two possible systems geometries, the one shown in FIG. 6 (corresponding to Littrow configuration) and another one similar to the Littman-Metcalf geometry for traditional EC QCLs (FIG. 7) are possible, among others. An embodiment of the broadly tunable quantum cascade laser is shown in FIG. 6. The laser comprises a gain medium 102, at least one collimating lens 104 a, b, at least one acousto-optic (AO) modulator 106, 108 and at least one highly reflective mirror 110.

In the Littman-Metcalf configuration, the output beam 124 exits from the module through the zero order diffraction of the grating created by the AOM 106. The configuration in FIG. 7 may have some advantage over that in FIG. 6. Using the undiffracted beam 120 (or the zero order diffraction beam in the traditional grating nomenclature) has the advantage of using one fewer collimating lens 104 b and associated advantage of reduced effort for aligning two lenses. The removal of one lens also reduces the overall cavity losses, thus will lead to a broader tuning range for the same QCL, chip. Therefore, as shown n FIG. 7, a collimating lens 104 b at one end can be replaced with a high reflective coating 130 at the gain media 102 of the QCL at that end.

Another variation of the configurations shown in FIGS. 6 and 7 is possible if continuous tuning is not required as may be the case when the tunable laser output interacts with a broad linewidth absorber, such as with chemical warfare agents, explosives vapors, and large molecule toxic industrial chemicals. This alternate variation, shown in FIGS. 8 and 9 correspond to the two configurations in FIGS. 6 and 7, respectively. The primary difference arises from the use of a standing acoustic wave 113 (as opposed to the traveling acoustic waves 115 a, b, in FIGS. 6 and 7). For example, the acoustic modulator 106 may not have an acoustic absorber 114 at one end. The diffraction of the laser beam from the standing acoustic wave is not accompanied by a Doppler frequency shift in the optical wave and thus a single resonant AOM 106 can be used, leading to substantial simplification of the optical setup and reduction in cost, as well.

As mentioned above, this configuration does not permit continuous wavelength tuning of the QCL because the standing wave nature of the acoustic grating requires an acoustic resonance in the modulator and thus only discrete acoustic wave frequencies are permitted, determined by the length of the AOM element. The acoustic resonance condition is written as shown in Equation 2 below: Nλ _(ac)=2L  Equation 2

where N is the number of acoustic resonances in the AOM 106, λ_(ac) is the acoustic wavelength and L is the length of the AOM 106. Thus the discrete permissible acoustic frequencies, v_(ac), for diffraction of the light beam are given by Equation 3 below:

$\begin{matrix} {v_{ac} = \frac{N\nu_{ac}}{2L}} & {{Equation}\mspace{14mu} 3} \end{matrix}$

where v_(ac) is the acoustic velocity in the AO material 107. FIGS. 10 and 11 show calculated discrete frequencies that are possible in a Ge AOM of 1 cm and 2 cm lengths, respectively.

Thus, for situations where continuous tuning not required, the standing wave AOM can provide an alternate and simpler solution since even for a 1 cm long AOM, over 100 discrete wavelengths can be obtained between tunable wavelengths of 7 μm and 11 μm.

Overall, the invention has several important advantages, over the grating, prism or filter based EC QCL, for practical field applications. For example, the tuning speed is improved. Typical RF modulation bandwidth for commercially available AOMs is in the range of tens of MHz. Therefore, the required switching time between arbitrary wavelengths of under 1 microsecond can easily be achieved. This switching time is at least five orders of magnitude faster than that for EC QCLs with a grating.

The flexibility of the wavelength control with AOMs also offers an interesting option of multi wavelengths operation when AOM wave is composed of several discrete frequencies. Each of the sound wave frequencies in this case will force the laser to operate at the corresponding optical frequencies. The multi wavelength operation can be important in spectroscopy when several spectral lines need to be tracked at the same time.

Also, the ruggedness is improved. In contrast to EC QCLs with a grating, wavelength tuning mechanism does not require any mechanical motion. The module can therefore be ruggedized to meet the most stringent requirements for field applications.

In addition, the yield (cost) is improved. The broadly tunable quantum cascade laser comprises a single QCL chip. The standard single-QCL fabrication process has a relatively high yield, exceeding 50 percent for watt-level LWIR devices. In case of QCL arrays, yield, however, quickly drops with increase in number of elements. For comparison, even a 10-element array processed from the same material would have a yield of less than 1 percent. Number of elements in a DFB QCL array required to cover the 7 to 11 micrometer tuning region can be as high as one hundred. As a consequence, DFB QCL arrays are projected to have a very low yield, making this technical approach impractical.

With regards to epi-down mounting, the present invention is compatible with QCL epi-down mounting that is now commonly employed to lower active region temperature of high average power devices. QCL arrays, on the other hand, are typically mounted epi-up to preserve individual electrical control for each of the emitters in the array. Overheating of the epi-up mounted array will make the goal of reaching high average optical power significantly more difficult, or even impossible.

With regards to reliability, all of the components in the proposed module have been in commercial use for some time and have proven their long-term reliability. The high power FP QCLs have been in commercial production for nearly a decade and the AOMs have been commercially deployed for several decades. Thus, the two critical components in the proposed configuration, namely FP QCLs and AOMs, represent mature technologies.

AOMs have been successfully used for rapid and random access discreet tuning of CO₂ lasers on their 9.6 micrometer and 10.6 micrometer laser bands and continuous tuning of Ti:sapphire lasers. Research also provides earlier data on tuning of dye lasers using acousto-optic modulators. However, because of the only recent development of QCLs, acousto-optic tuning of QCLs has not been attempted.

This technology may be applicable to the interband cascade lasers as well.

EXAMPLES

Reported is the first operation of tunable external cavity quantum cascade lasers with emission wavelength controlled by an AOM. A long-wave infrared quantum cascade laser wavelength is tuned from approximately 8.5 micrometer to approximately 9.8 micrometer when the AOM frequency is changed from 41 MHz to 49 MHz. The laser delivered over 350 mW of average power in the center of the tuning curve in a linewidth of 4.7 cm⁻¹. Measured wavelength switching time from the edge of the tuning curve to the center of the tuning curve is less than 1 microsecond. Initial spectral measurements of infrared absorption features of gaseous isopropyl alcohol were carried out, which demonstrate a capability of obtaining complete spectral data from approximately 8.5 micrometer to approximately 9.8 micrometer in less than 20 microseconds. The demonstration paves a way for a new generation of tunable QCLs providing ruggedness, fast tuning and high power capability in the infrared spectral region.

The AOM 106 tuned QCL configuration is shown in FIG. 12. The piezoelectric transducer 112 of the AOM creates a traveling acoustic wave 115 in the germanium crystal transparent medium 107. The sound wave, in turn, produces a spatial periodic variation in refractive index of the crystal, i.e. it creates a phase grating. The undiffracted optical beam 120 incident on the AOM 106 experiences a wavelength dependent deflection. The diffracted beam 122 subsequently reflects back from the mirror 110 to the AOM 106 and couples back into the gain medium 102, completing the wavelength-dependent feedback loop. The output optical beam 124 from the gain medium 102 emerges from the other side 105 (distal facet) of the laser.

The period of the acoustic grating is electronically controlled, which allows for the rapid change of the diffracted wavelength. Typical modulation bandwidth for commercially available AOMs is in the range of tens of MHz. Hence, the required switching time, between arbitrary wavelengths, of under 1 microsecond can be achieved. This switching time is at least five orders of magnitude faster than that for EC QCLs with a grating. In addition, in contrast to EC QCLs with a grating, wavelength tuning mechanism does not require any mechanical motion. The module can therefore be ruggedized to meet the most stringent requirements for field applications.

The frequency of the AOM used in this example was adjustable in the range from 35 MHz to 55 MHz. FIG. 13 shows a measured tuning curve for a 9 micrometer QCL, with the QCL design described in A. Lyakh R. Maulini, A, Tsekoun, R. Go, and C. K, N. Patel, “Multiwatt long wavelength quantum cascade lasers based on high strain composition with 70% injection efficiency”, Opt. Expr. 22, 24272 (2012) (which is incorporated here by this reference), operating in a quasi-CW mode (350 nanosecond pulses and 50 percent duty cycle) as AOM frequency was changed from 41.7 MHz to 48.5 MHz (approximately ⅓ of the available frequency range). AOM input power was fixed at 35 W during the testing. The emission wavelength for the EC laser tuned from 1020 cm⁻¹ (9.8 micrometer) to 1170 cm⁻¹ (8.5 micrometer). The tuning range available from a single laser can be increased using broader gain QCL chips.

Measured QCL emission spectrum linewidth for AOM central frequency of 45 MHz was 4.7 cm⁻¹. The linewidth can be changed by aligning the system so that it operates at another AOM central frequency (different Bragg condition). Employment of a higher AOM frequency will result into a larger number of illuminated acoustic grating periods and, therefore, to a proportionately narrower width. This effect is illustrated in FIGS. 14 and 15. The increase in the central AOM frequency from 35 MHz to 55 MHz leads to a reduction in emission linewidth from 5.7 cm⁻¹ to 3.9 cm⁻¹.

As shown in FIG. 17, the response time of the change in the optical wavelength with a change in the AOM frequency has two components: (1) propagation time of the acoustic wave from the acoustic transducer 112 to the edge of the optical beam L going through the AOM, and (2) the propagation time for the acoustic wave across the optical beam D_(B). The first “delay” is the latency time and does not represent the response time of the change the optical wavelength when AOM frequency is changed. The latency time can be shortened, almost arbitrarily, by reducing the distance between the acoustic transducer and the position of the optical beam. The actual response time, therefore, is determined by the acoustic wave transit time across the optical beam. For the present case, the time it takes an acoustic wave to propagate from the piezoelectric transducer across the germanium material to the edge of the optical beam t₁ and time for the acoustic wave to cross the optical beam is T₂. The time t₂ is the actual response time of the AOM for changing the optical wavelength. As mentioned above, t₁ can be shortened to almost zero. However, there are limitations on how short t₂ can be. If t₂ is made too short by making the optical beam diameter small, the optical wave will interact with a fewer number of acoustic waves and therefore the linewidth of the output will increase. The linewidth of the optical output can be reduced by making the optical beam diameter larger, but that occurs at the expense of the response time. The optimal linewidth/response time balance is application driven.

FIGS. 16A and 16B show measured response time of the system as AOM frequency switches from 35 MHz (outside of the tuning curve) to 45 MHz (center of the tuning curve). FIG. 14B shows the AOM frequency control signal and FIG. 14A shows the optical signal from the laser, which consists of pulses of duration 350 nanosecond pulses with a pulse repetition period of approximately 700 nanoseconds.

AOM frequency is initially equal to 35 MHz, outside of the tuning curve of the laser. The optical signal from the laser is at zero level since the Bragg condition is not satisfied anywhere in the laser gain spectrum. AOM frequency is abruptly changed to 45 MHz at time t=0. AOM frequency of 45 MHz corresponds to the peak of the gain curve. It takes approximately t₁=L/v_(s)=1.25 microseconds for the acoustic signal with the new frequency to reach the area where the optical beam is incident on the crystal (v_(s) is sound velocity and L is defined in FIG. 17). When the sound signal with the new frequency reaches that area, the optical signal starts growing. It reaches its maximum value at t₂=550 nanoseconds+t₁, in a good agreement with the calculated travelling time across the beam area t₂=D_(B)/v_(s)=3.4 millimeter/(5.5 millimeter/microsecond) approximately 600 nanoseconds (FIG. 16A). The AOM frequency switches back to 35 MHz at t=3 microseconds. The process is repeated: it takes about t₁+t₂=1.8 microseconds for the laser to completely shut down (FIG. 16B).

As mentioned above, the latency time t₁ is proportional to L and can be almost arbitrarily minimized by designing the AOM so that the piezoelectric transducer generating the sound wave is positioned closer to the beam area (L in FIG. 17 as short as possible). Time t₂, on the other hand, is fixed for the same choice of material and beam size. It is, however, already well below 1 microsecond. Thus, if the AOM frequency was changed between two values, for example, f₁ and f₂, the laser wavelength will switch between two corresponding values in a time less than 1 microsecond, the latency time notwithstanding.

To demonstrate the rapid spectral measurement capability for the AOM-controlled EC QCLs, a transmission absorption spectrum of isopropyl alcohol vapor that has a broad infrared absorption spectrum. The measurement was done in the sweep mode when acousto-optic frequency was varied from 42 MHz to 48 MHz. FIG. 18 shows the comparison between an IPA spectrum generated using the HITRAN database and collected experimental data. The experimental data were captured in a single AOM frequency sweep from 42 MHz to 48 MHz that took less than 20 microseconds.

In conclusion, the first data for AOM-controlled external cavity QCLs is presented. These devices offer the advantage of very fast tuning capability with spectral measurement me of under 20 seconds. The configuration does not involve any moving parts and therefore can be ruggedized for demanding field applications.

The present invention may also be applied to ring laser geometry as shown in FIGS. 19-21. The ring laser configuration has been extensively studied in the near infrared lasers. Compared to traditional standing wave cavity lasers, described in other parts of this patent application, ring cavity lasers can operate in either of the counter propagating directions or both. This capability offers advantages including the capability of unidirectional laser oscillation with at least the following advantages: (1) ring laser operating in a unidirectional mode, eliminates the spatial hole burning effect present in traditional cavity lasers and extracts power from the gain medium uniformly, making it homogeneous, which increases mode competition between adjacent longitudinal modes and affords the possibility of single mode operation even at high power outputs; (2) uniform extraction of power without spatial hole burning results in higher single mode power output compared to that possible with standing wave configuration; (3) ring cavity operation is less sensitive to small misalignments; and (4) ring cavity laser operated in a unidirectional mode can be less sensitive to optical feedback from external elements.

As shown in FIGS. 19 and 20, the ring laser geometry may comprise the gain medium 102 of the laser having anti-reflection coatings 132 a, b at opposite ends of the gain medium 102, a pair of collimating lenses 104 a, b at opposite ends of the gain medium 102, an AOM 106 powered by a radiofrequency generator 125, and a plurality of highly reflective mirrors 110 a-c. The highly reflective mirrors are positioned so that one mirror 110 a receives the diffracted beam 122 from the AOM 106 and deflects the diffracted beam 122 to the second mirror 110 b positioned on the opposite side of the gain medium 102, which in turn deflects the diffracted beam 122 to the third mirror 110 c, which deflects the diffracted beam 122 through one of the collimating mirrors 104 b and back to the gain medium 102. In some embodiments, the AOM 106 may have an acoustic absorber 114 opposite the piezoelectric transducer 112 so as to generate a traveling acoustic wave 115 through the material 107 (FIG. 19). In other embodiments, the AOM 106 may not have the acoustic absorber so as to create a standing wave 113 (FIG. 20).

In some embodiments, as shown in FIG. 21, the laser may have a high reflective coating 130 at one end or facet 105 (the distal end) of the gain medium 102 and only one collimating lens 104 at the opposite end or facet 103 (the near end) of the gain medium 102. The beam is emitted from one end or facet 103 of the gain medium 102, through the collimating lens 104, and passes through the AOM 106. The beam is diffracted through the AOM 106 and is directed to the first mirror 110 a. Through a series of reflections off of the other mirrors 110 b, 110 c, the diffracted beam 122 returns to the laser at the opposite end or facet 105 of the gain medium 102 having the high reflective coating 130.

In embodiments utilizing travelling waves, a second AOM 108 may be used in the opposite orientation relative to the first AOM 106, and in between the first AOM 106 and the first mirror 110 a, so that the travelling waves from each AOM 106, 108 travel in opposite directions so that the Doppler shift in the frequency of the diffracted laser beam created in the first AOM 106 is cancelled out by the complimentary Doppler shift introduced by the second AOM 108.

QCLs or gain medium 102 are fabricated out of multilayer structures comprising AlInAs/GaInAs with controlled and predetermined fractions of Al and In in AlInAs and controlled and predetermined fractions of Ga and In in GaInAs. The refractive index, n, of this multilayer structure end facet is approximately 3.4. A pristine end facet surface 103, 105 has a reflectivity given by:

$\begin{matrix} {R = \left( \frac{n - 1}{n + 1} \right)^{2}} & {{Equation}\mspace{14mu} 4} \end{matrix}$

Where n is approximately 3.4, R is 0.3.

Thus, for a QCL chip with both uncoated facets, the device will lase as a Fabry-Perot cavity formed by the 30% reflection from the end facets 103, 105. For Fabry-Perot configuration QCLs with both facets 103, 105 uncoated, the laser output will occur from both ends, and the output wavelength spectrum will be broad, as determined by the gain bandwidth of the QCL, which will be typically 100 cm⁻¹ to 200 cm⁻¹. For such lasers to be useful devices, generally one facet (e.g. facet 105) is coated with a totally reflective or high reflectivity coating 130, either multilayer dielectric coating or more generally evaporated gold coating. For the latter case, a thin layer of AlN or similar high resistivity material, transparent for the wavelengths of interest, is deposited on the end facet before the gold coating to make sure that the gold coating does not electrically short the QCL. A high reflectivity coating 130 will reflect substantially all of the laser radiation. Now, the Fabry-Perot cavity QCL with the high reflectivity coating 130 will emit laser radiation only from the uncoated facet (e.g. facet 103).

Lasing operation and the power output from a Fabry-Perot cavity QCL is determined by the amount of optical gain produced in the chip (gain medium 102) and the total optical loss. For a chip (gain medium 102) with one facet high reflectivity (HR) coated 130, the loss consists of the inherent loss within the QCL chip and the 70% loss through the uncoated facet (exit facet). Of course, in order for the chip (gain medium 102) to lase, the gain must exceed the total loss. However, it is recognized by scientists, that the 30% reflectivity of the uncoated exit facet is too high to extract optimum amount of power from the laser cavity. Thus, multilayer dielectric coating may be deposited on the exit facet such that the reflectivity is less than 30%, so that more power can be extracted. This type of coating is referred to as controlled reflectivity anti-reflection coating or partially reflective coating 133. The exact amount of reflectivity will depend on the details of the amount of gain produced by the QCL chip and other losses in the cavity. In the preferred embodiment, controlled reflectivity anti-reflection coating 133 is applied such that reflectivity may be from approximately 10% to approximately 30%. In some embodiments, the reflectivity may be from approximately 15% to approximately 25%. The various coating techniques, i.e. high reflective coating 130, anti-reflection coating 132 a, b, and controlled reflectivity anti-reflection coating 133 can be applied in any of the embodiments described herein, including, but not limited to, what is shown in the figures.

In the patent application under consideration, the lasing cavity is not the Fabry-Perot cavity formed by the end facets. As a matter of fact, it is necessary to suppress Fabry-Perot cavity lasing because tunable wavelength output is desired (as opposed to a broad band output, characteristic of a Fabry-Perot cavity). Thus the lasing cavity comprises reflection from one of the facets of the QCL chip (e.g. the distal facet 105) and the reflecting mirror 110 c beyond the acousto-optic modulator 106. The acousto-optic modulator 106 determines which specific wavelength is reflected back into the QCL. In order to suppress Fabry-Perot cavity lasing by the QCL, the near facet (e.g. facet 103) of the QCL chip is anti-reflection (AR) coated 132 a with multilayer dielectric coatings. Now, in order to extract the maximum amount of tunable power (from the distal end of the QCL chip), the total gain in the chip and the total losses may be considered. In general, in spite of the losses in the lasing cavity formed by travel through the acousto-optic modulator, reflection from the fixed mirror and the transmission losses through the exit face, the 30% reflectivity of native facet is too high. So, just as with optimizing power output from a Fabry-Perot cavity laser, described above, we deposit a controlled reflectivity anti-reflection coating 133 on the exit facet (e.g. facet 105). The reflectivity of this coating is generally less than that of a native facet. Again, the optimum reflectivity of the partially reflective anti-reflection coating 133 will depend on the details of the system.

In some embodiments, the radiofrequency generator 125, 126 operatively connected to the acousto-optic modulator 106, 108 may be configured to generate two radiofrequencies simultaneously. In some embodiments, one of the first or second radiofrequencies may be fixed and the other of the first or second radiofrequencies may be varied to simultaneously generate two laser wavelengths, one which is fixed and the other which is tuned. Traditional tunable lasers, using diffraction gratings (FIGS. 3 and 4), prisms (FIG. 5) or filters (FIG. 6) are not able to support simultaneous operation on two wavelengths, because for any angular setting of the grating, prism or the filter, only one wavelength can resonate inside the optical cavity.

In some embodiments, one of the first or second radiofrequencies may be fixed and the other of the first or second radiofrequencies may be varied and temporally switched to simultaneously generate two laser wavelengths, one which is fixed in wavelength and unswitched, and the other which is tuned and is temporally switched. Such operation provides the capability of pumping (exciting) the medium at on wavelength (tuned and switched) and monitoring the response of the medium at the other wavelength that is tuned but not switched. These measurement techniques are called “pump-probe” studies.

In some embodiments, one of the first or second radiofrequencies may be fixed and temporally switched, and the other of the first or second radiofrequencies may be varied and temporally switched to simultaneously generate two laser wavelengths, one which is fixed in wavelength and temporally switched, and the other which is tuned and is temporally switched. This arrangement will provide yet another way of carrying out pump-probe studies of a medium in the infrared.

The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention not be limited by this detailed description, but by the claims and the equivalents to the claims appended hereto. 

What is claimed is:
 1. A laser, comprising: a) a quantum cascade laser gain medium having a first end and a second end; b) a collimating lens adjacent to the first end of the gain medium and receiving an undiffracted output beam from the gain medium, the undiffracted beam defining a first path; c) an anti-reflection coating at the first end of the gain medium; d) an acousto-optic modulator comprising germanium, wherein the acousto-optic modulator is adjacent to the first collimating lens and opposite the gain medium, the acousto-optic modulator receiving the undiffracted output beam from the first collimating lens and subsequently emitting a diffracted beam along a second path; e) a reflective mirror positioned to receive the diffracted beam and reflect the diffracted beam back along the second path to the acousto-optic modulator through the first path and back to the gain medium to amplify a wavelength tuned beam; and f) a radiofrequency generator operatively connected to the acousto-optic modulator, the radiofrequency generator configured to generate a first radiofrequency and a second radiofrequency simultaneously, wherein the first radiofrequency is fixed and is temporally switched, and the second radiofrequency is varied and is temporally switched to simultaneously generate a first laser wavelength that is fixed in wavelength and temporally switched, and a second laser wavelength that is tuned and is temporally switched.
 2. The laser of claim 1, further comprising a reflective coating at the second end of the gain medium.
 3. The laser of claim 1, wherein the acousto-optic modulator comprises a first end, a second end opposite the first end, a transparent material between the first end and the second end of the acousto-optic modulator, and a piezoelectric transducer at the first end of the acousto-optic modulator.
 4. The laser of claim 3, wherein the acousto-optic modulator comprises an acoustic absorber at the second end of the acousto-optic modulator.
 5. The laser of claim 3, wherein the acousto-optic modulator comprises: a) a first facet adjacent to the first and second ends of the acousto-optic modulator, the first facet having a first anti-reflection coating at a desired optical wavelength, and b) a second facet opposite the first facet and adjacent to the first and second ends of the acousto-optic modulator, the second facet having a second anti-reflection coating at a desired optical wavelength.
 6. The laser of claim 1, wherein the laser is an infrared quantum cascade laser or an infrared interband cascade laser.
 7. The laser of claim 1, further comprising a plurality of reflective mirrors to form a ring laser resonator.
 8. A laser, comprising: a) a quantum cascade laser gain medium having a first end and a second end; b) a collimating lens adjacent to the first end of the gain medium and receiving an undiffracted output beam from the gain medium, the undiffracted beam defining a first path; c) an anti-reflection coating at the first end of the gain medium; d) an acousto-optic modulator adjacent to the first collimating lens and opposite the gain medium, the acousto-optic modulator receiving the undiffracted output beam from the first collimating lens and subsequently emitting a diffracted beam along a second path; e) a reflective mirror positioned to receive the diffracted beam and reflect the diffracted beam back along the second path to the acousto-optic modulator through the first path and back to the gain medium to amplify a wavelength tuned beam; and f) a radiofrequency generator operatively connected to the acousto-optic modulator, the radiofrequency generator configured to generate a first radiofrequency and a second radiofrequency simultaneously, wherein the first radiofrequency is fixed and is temporally switched, and the second radiofrequency is varied and is temporally switched to simultaneously generate a first laser wavelength that is fixed in wavelength and temporally switched, and a second laser wavelength that is tuned and is temporally switched, wherein the acousto-optic modulator comprises a first end, a second end opposite the first end, a transparent material between the first end and the second end of the acousto-optic modulator, and a piezoelectric transducer at the first end of the acousto-optic modulator, wherein the acousto-optic modulator comprises an acoustic absorber at the second end of the acousto-optic modulator.
 9. A laser, comprising: a) a quantum cascade laser gain medium having a first end and a second end; b) a collimating lens adjacent to the first end of the gain medium and receiving an undiffracted output beam from the gain medium, the undiffracted beam defining a first path; c) an anti-reflection coating at the first end of the gain medium; d) an acousto-optic modulator adjacent to the first collimating lens and opposite the gain medium, the acousto-optic modulator receiving the undiffracted output beam from the first collimating lens and subsequently emitting a diffracted beam along a second path; e) a reflective mirror positioned to receive the diffracted beam and reflect the diffracted beam back along the second path to the acousto-optic modulator through the first path and back to the gain medium to amplify a wavelength tuned beam; and f) a radiofrequency generator operatively connected to the acousto-optic modulator, the radiofrequency generator configured to generate a first radiofrequency and a second radiofrequency simultaneously, wherein the first radiofrequency is fixed and is temporally switched, and the second radiofrequency is varied and is temporally switched to simultaneously generate a first laser wavelength that is fixed in wavelength and temporally switched, and a second laser wavelength that is tuned and is temporally switched, wherein the acousto-optic modulator comprises a first end, a second end opposite the first end, a transparent material between the first end and the second end of the acousto-optic modulator, and a piezoelectric transducer at the first end of the acousto-optic modulator, wherein the acousto-optic modulator comprises a first facet adjacent to the first and second ends of the acousto-optic modulator, the first facet having a first anti-reflection coating at a desired optical wavelength, and a second facet opposite the first facet and adjacent to the first and second ends of the acousto-optic modulator, the second facet having a second anti-reflection coating at a desired optical wavelength. 